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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id a9-20020aa78e89000000b005379fb50ff0sm216131pfr.50.2022.08.25.16.43.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 25 Aug 2022 16:43:31 -0700 (PDT) Date: Thu, 25 Aug 2022 23:43:27 +0000 From: Sean Christopherson To: "Kirill A. Shutemov" Cc: Chao Peng , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, qemu-devel@nongnu.org, Paolo Bonzini , Jonathan Corbet , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H . Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Mike Rapoport , Steven Price , "Maciej S . Szmigiero" , Vlastimil Babka , Vishal Annapurve , Yu Zhang , "Kirill A . Shutemov" , luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com, ak@linux.intel.com, david@redhat.com, aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, Quentin Perret , Michael Roth , mhocko@suse.com Subject: Re: [PATCH v6 6/8] KVM: Handle page fault for private memory Message-ID: References: <20220519153713.819591-1-chao.p.peng@linux.intel.com> <20220519153713.819591-7-chao.p.peng@linux.intel.com> <20220819004018.mgdvxhl6dj3ujl3f@box.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220819004018.mgdvxhl6dj3ujl3f@box.shutemov.name> ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661471013; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=CrkUDi4mYcC3qOEbpgTO1y4xAYzFYl9qfJxBFAO1WQU=; b=VbWahMKMGIMkgYx76zcet4oekHU5nYs4aDeGy1SujpYq9EiOXeMqbKnJLZR6sJpOQLLVQY XkvSND9cChOwRMqPdRNfjJ2VpEHVJmPz1/kcrP+kKtA194mcUle3L4WcQ/f/o+nzJ0Lf86 Au3K5FS1DFHqY7q+LpizO2uW+ku3vnU= ARC-Authentication-Results: i=1; imf09.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=oH89fuC7; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf09.hostedemail.com: domain of seanjc@google.com designates 209.85.214.182 as permitted sender) smtp.mailfrom=seanjc@google.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661471013; a=rsa-sha256; cv=none; b=yKj1V/haKq+4lIwOCstoS50DokYDNwDWlUVSyYUQGbKgVOCoDiuwFId4UbFNc3Xbtz9frR 7BLB+E7RDLIy1hklJWH5vLZ6H8alCVtsJ9m3P5XkGB/G/nI91TYAPDqvJ6kW09BvzCAJPJ 0Er5EOUG5OJU1SWuvRHo7sj7jYsZij8= Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=oH89fuC7; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf09.hostedemail.com: domain of seanjc@google.com designates 209.85.214.182 as permitted sender) smtp.mailfrom=seanjc@google.com X-Rspam-User: X-Rspamd-Queue-Id: 7940A14002C X-Rspamd-Server: rspam10 X-Stat-Signature: r4ur98em33qxd1hhfg7kzxtuais7hexq X-HE-Tag: 1661471013-8141 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Aug 19, 2022, Kirill A. Shutemov wrote: > On Fri, Jun 17, 2022 at 09:30:53PM +0000, Sean Christopherson wrote: > > > @@ -4088,7 +4144,12 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault > > > read_unlock(&vcpu->kvm->mmu_lock); > > > else > > > write_unlock(&vcpu->kvm->mmu_lock); > > > - kvm_release_pfn_clean(fault->pfn); > > > + > > > + if (fault->is_private) > > > + kvm_private_mem_put_pfn(fault->slot, fault->pfn); > > > > Why does the shmem path lock the page, and then unlock it here? > > Lock is require to avoid race with truncate / punch hole. Like if truncate > happens after get_pfn(), but before it gets into SEPT we are screwed. Getting the PFN into the SPTE doesn't provide protection in and of itself. The protection against truncation and whatnot comes from KVM getting a notification and either retrying the fault (notification acquires mmu_lock before direct_page_fault()), or blocking the notification (truncate / punch hole) until after KVM installs the SPTE. I.e. KVM just needs to ensure it doesn't install a SPTE _after_ getting notified. If the API is similar to gup(), i.e. only elevates the refcount but doesn't lock the page, then there's no need for a separate kvm_private_mem_put_pfn(), and in fact no need for ->put_unlock_pfn() because can KVM do set_page_dirty() and put_page() directly as needed using all of KVM's existing mechanisms.